1. Field of the Invention
The present invention relates to a receiver and, more particularly, to a superheterodyne type radio receiver.
2. Description of the Related Art
Generally, a radio receiver includes a filter for filtering out noise contained in a received radio signal. It is desirable with this kind of filter to reduce variation in characteristic ascribable to, e.g., scattering among its constituents, temperature variation, and power source voltage variation. It is also desirable that the filter be operable with high stability and accuracy. A stable and accurate gyrator filter circuit is disclosed in, e.g., Japanese laid-open patent application heisei 2-274115 assigned to the same assignee as the present application.
Referring to FIG. 1, the gyrator filter circuit includes a first and a second gyrator filter 1 and 2, respectively. The first gyrator filter 1 has a preselected frequency characteristic for an input signal IN when a circuit current 11 fed to the filter 1 has a preselected value. When the value of the circuit current I1 varies, the frequency characteristic of the current I1 varies substantially in parallel in the high-and-low direction of frequency. The filter 1 produces an output signal OUT by filtering the input signal IN.
The second gyrator filter 2 is identical in function and characteristic with the first gyrator filter 1. A reference signal generator 3 generates a reference signal Vf having a frequency relating to the cut-off range of a required frequency characteristic. The reference signal Vf is input to the second filter 2. The filter 2 applies its output signal VF to an amplitude detector 4 by filtering the reference signal Vf.
The amplitude detector 4 detects the output signal VF of the second filter 2 while feeding a voltage VD corresponding to the level of the signal VF to a comparator 5. The comparator 5 includes an operational amplifier (OP AMP) 6. The comparator 5 compares the voltage VD output from the amplitude detector 4 with a DC reference voltage VR. The result of comparison is fed from the comparator 5 to a current source 7 as a comparison signal VC.
The current source 7 outputs, in response to the comparison signal VC, a circuit current I2 causing the voltage VD output form the amplitude detector 4 to vary toward the DC reference voltage VR, and a circuit current I1 equal to the circuit current I2. The current source 7 feeds the circuit currents I1 and I2 to the first and second gyrator filters 1 and 2, respectively.
In the above filter circuit, assume that the frequency characteristics of the two gyrator filters 1 and 2 are shifted to the higher frequency side from their stable conditions due to, e.g., scattering among constituent parts, temperature variation, or power source voltage variation. Then, the level of the signal VF output from the second filter 2 increases, and so does the level of the voltage VD output from the amplitude detector 4. Because the voltage VD rises above the DC reference voltage VR, the OP AMP 6 lowers the level of the comparison signal VC. Because the level of the comparison signal VC is lowered, the current source 7 lowers the circuit currents I1 and I2 and then delivers them to the filters 1 and 2, respectively. The decrease in the circuit currents I1 and I2 causes the frequency characteristics of the filters 1 and 2 to shift to the lower frequency side, i.e., to the original frequency characteristics.
In the conventional gyrator filter circuit, a negative feedback loop to the second filter 2 is formed in order to insure the stable and accurate frequency characteristic of the first filter 1. With this configuration, it is possible to reduce the variation of filter circuit characteristic ascribable to scattering among the constituent parts of the two filters themselves, temperature variation, power source voltage variation, and so forth.
However, the conventional gyrator filter circuit gives no consideration to the variation of the characteristic of the entire radio receiver including the filter circuit. This brings about a problem that a received signal misses the frequency band of an intermediate frequency (IF) filter and is attenuated due to the deviation of the output frequency of a local oscillator, i.e., a frequency offset, deteriorating receipt sensitivity to a critical degree.
To detect an error of a filter output, it is necessary to compare the filter output with some reference. The prerequisite is that the reference be constant under any possible condition in order to insure accurate error detection. The conventional filter circuit detects the error of the filters by using the level voltage output from the amplitude detector. To implement the level voltage, the conventional filter circuit includes the reference signal generator. However, accurate adjustment of the filters is not achievable unless the oscillation frequency of the reference signal output from the reference signal generator remains constant, i.e., unless the reference signals is highly accurate. The reference signal is usually output from a quartz oscillator. Although the accurate reference signal may be implemented if the temperature variation and production errors of the quartz oscillator are reduced, the resulting quartz oscillation is extremely expensive.
Today, in parallel with the increase in the frequency of a channel frequency, the pass band of an IF band pass filter is increasing in order to reduce the influence of a frequency offset ascribable to the variation of a local oscillation frequency caused by varying temperature. This, however, cannot be done without degrading the carrier-to-noise (CN) ratio of a receiver.